For a liquid-crystal display element to function, the direction of orientation of liquid-crystal molecules and the direction of polarization of the polarizer must be set in a particular relation to each other. In liquid-crystal display element technologies, LCDs using a TN (Twisted Nematic) type liquid-crystal were the first to be put into practical use. Recently, LCDs using a VA (vertical Alignment) type liquid-crystal, an IPS (Inplane Switching) type liquid-crystal etc., were put into practical use. Although a technical explanation is omitted, in an LCD using such TN-type liquid-crystal panel, liquid-crystal molecules are provided between two upper and lower orientation films having respective rubbing directions on the inner surfaces of the substrates of the liquid-crystal panel. This means that the liquid-crystal molecules are twisted by 90 degrees along the optical axis, so that when a voltage is applied, the liquid-crystal molecules are aligned in a direction perpendicular to the orientation films. However, in the case where the LCD is designed to allow images as seen from right and left sides of a display screen as those view from directly in front of the display screen, the direction of rubbing on the orientation film at the viewing-side must be 45 degrees (the rubbing direction of the other orientation film being 135 degrees). It is therefore necessary that the polarizing sheets made from the polarizing composite films to be laminated respectively on the front and back sides of the liquid-crystal panel with polarizers respectively oriented in directions inclined respectively by 45 degrees with respect to a lengthwise or widthwise direction of the display screen so as to conform to the rubbing directions.
Therefore, in a polarizing sheet for use in producing a liquid-crystal display element of a TN-type liquid-crystal panel, it is required that the optical film is punched or cut into a rectangular-shaped sheet having a long side or a short side determined in accordance with the size of the TN liquid-crystal panel, and inclined by 45 degrees with respect to the orientation direction of the polarizer produced by stretching in the lengthwise or widthwise direction. This is described in Japanese Laid-Open Patent Publication No. JP 2003-161935A or Japanese Patent No. 3616866 B. The width or the short side dimension of the optical film sheet to be processed into the rectangular shape is smaller than the width of the optical film.
The punching or cutting of the optical film into the rectangular-shaped sheet may be collectively referred to as “individualized sheets” or “a method and system for manufacturing individualized sheets” for liquid-crystal display elements. The optical film sheet thus punched or cut is produced by integrally punching or cutting not only the surface-protection film but also the carrier film protecting the exposed surface of the adhesive layer in the polarizing composite film. The integrally punched-out or cut carrier film sheet may be referred to as “separator,” rather than “carrier film sheet” because it is not serving as a transport medium. Thus, the manufacturing process of the liquid-crystal display elements includes the first step of peeling the separator from each of the optical film sheet to have the adhesive layer in the polarizing sheet exposed. Subsequently, the polarizing sheets each having the adhesive layer exposed are conveyed one-by-one, for example, under a vacuum suction irrespective of whether the surface-protection films are laminated on the polarizing sheets or not, and laminated to respective ones of a plurality of liquid-crystal panels. According to the aforementioned manufacturing process of the liquid-crystal display elements, it has been required that the integrally punched-out or cut sheet from the optical film is in the form of an individualized sheet having four trimmed sides and a certain level of stiffness of less deflection or bend and which can be conveyed and laminated easily. During the initial period in the history of the manufacturing process of the liquid-crystal display elements, the optical film sheet or a polarizing sheet comprised in such optical film sheet was generally known as a “polarizing plate” which is still used as a common name.
In the manufacturing process of TN-type liquid-crystal display elements, an optical film fed out from a roll of continuous web is integrally and sequentially punched-out or cut in a direction transverse to the feed direction. However, in this case, it is impossible to obtain a finished liquid-crystal display element simply by sequentially laminating the sheets formed to respective ones of a plurality of liquid-crystal panels. This is because the sheets each formed with a long or short side extending in a direction 45 degrees cannot be laminated sequentially to respective ones of the liquid-crystal panels with the same posture. Therefore, to provide a finished liquid-crystal display element by transporting a polarizing sheet to a position for lamination with a liquid-crystal panel, and then laminating the polarizing sheet to the liquid-crystal panel, an optical film in the form of a continuous web having a width greater than a long side of a liquid-crystal panel is fed out in a lengthwise direction, and each of the sheets are punched-out at an angled direction of 45 degrees with respect to the lengthwise direction, using, for example, a die into a plurality of individual polarizing sheet, and appropriately fed to the lamination process with the liquid-crystal panel, as shown in the Japanese Laid-Open Patent Publication No. 2003-161935A or Japanese Patent No. 3616866B. Alternatively, manufacturing methods for liquid-crystal display elements are provided wherein one of the methods uses a single sheet formed from the continuous web of the optical film, the single sheet having a substantially large width by punching or cutting the optical film in a direction 45 degrees inclined with respect to the lengthwise direction as shown in the Japanese Patent Publication No. 62-14810B or Japanese Laid-Open Patent Publication No. 55-120005A. The liquid-crystal display element is produced by making the elongated optical film having a width of the liquid-crystal panel thus formed into a continuous roll, feeding the elongated optical film from the continuous roll, forming a plurality of sheets having required length by cutting the film in the widthwise direction with respect to its feed direction and laminating the plurality of the polarizing sheets contained in the sheet to respective one of liquid-crystal panels W sequentially conveyed. At any rate, the above techniques are not beyond the system for manufacturing individualized sheets based on the premise of TN-type liquid-crystal display elements.
Japanese Patent Publication No. 62-14810B discloses, prior to the VA-type liquid-crystal and the IPS-type liquid-crystal being brought into practical use, an apparatus to produce a liquid-crystal panel. Japanese Patent Publication No. 62-14810B further discloses a technique of continuously feeding out an optical film which comprises a polarizing composite film (in Japanese Patent Publication No. 62-14810B, referred to as “elongated polarizing plate”) and a separator for protecting an adhesive layer on the polarizing composite film, “cutting only the polarizing plate 4 and the adhesive layer 5 while leaving the separator 6 uncut (hereinafter referred to as “half-cut”),” removing defective polarizing sheets formed in the course of the feeding, sequentially laminating the peeled sheets onto a plurality of liquid-crystal panels (referred to as “liquid-crystal cells”) for constituting small-size display screens of electronic calculators or the like, while peeling the separator from the polarizing sheets which have been retained on the separator. The apparatus is a labeler unit which produces an LCD using the TN-type liquid-crystal. Thus, the optical film to be used, of course, must be an elongated sheet produced from an optical film having substantially large width by cutting it in a direction 45 degrees oblique to the longitudinal direction of the optical film with a width corresponding to the width of the liquid-crystal panel. Therefore, this apparatus cannot be applied directly to a manufacturing apparatus adapted to perform steps of continuously forming a plurality of polarizing sheets from an optical film and laminating respective sheets to respective ones of the liquid-crystal panel using VA-type or IPS-type liquid-crystal to produce liquid-crystal display elements because of the width of optical film required.
Japanese Laid-Open Patent Publication No. 55-120005A discloses, prior to the VA-type liquid-crystal and the IPS-type liquid-crystal being brought into practical use, an apparatus to produce a liquid-crystal display element by sequentially laminating a plurality of sheets formed into a required length to a plurality of liquid-crystal panels while continuously feeding out an optical film containing a polarizing composite film. In the manufacturing method disclosed an adhesive layer is formed on a large-width polarizing composite film. A plurality of elongated polarizing composite film sheets having a required width are cut out from the large-width polarizing composite film. These sheets are laminated to separately prepared conveyance medium (i.e., carrier film) subjected to a releasing treatment to produce an optical film. Then, the optical film is half-cut in a vertical direction by two knives provided with a required distance with respect to a longitudinal direction, leaving the conveyance medium uncut, the optical film sheet is continuously formed separated from each other on the conveyance medium, and the plurality of formed sheets are sequentially laminated to respective ones of the liquid-crystal panels being conveyed to manufacture the liquid-crystal element. This apparatus is also based on the use of an elongated polarizing sheet which is cut in a direction 45 degrees oblique to the stretching direction of the polarizing composite film with a width corresponding to the width of the liquid-crystal panel, so that it cannot be applied directly to a manufacturing apparatus adapted to VA-type or IPS-type liquid-crystal to produce liquid-crystal display elements.
Automation of manufacturing process for liquid-crystal display elements using individualized sheets is described, for example, in Japanese Laid-Open Patent Publication No. 2002-23151A. Flexible individualized sheets tend to be bowed or warped due to curves or distortion of its edge, and thus it is a serious technical impediment to accuracy and speed in registration and lamination with liquid-crystal panels. Thus, it will be understood that the individualized sheet is required to have a certain level of thickness and stiffness to facilitate registration and lamination with liquid-crystal panels typically in transportation under suction. For example, the disclosures in the Japanese Laid-Open Patent Publication No. 2004-144913A, Japanese Laid-Open Patent Publication No. 2005-298208A or Japanese Laid-Open Patent Publication No. 2006-58411A disclose measures for addressing such technical problems.
On the other hand, the VA-type and IPS-type liquid-crystal panels are not designed to arrange liquid-crystal molecules in twisted orientations. Thus, when producing liquid-crystal display element using these types of liquid-crystal panels, there is no need to have the polarization axis of the polarizing sheet oriented at 45 degrees. Each of these liquid-crystal display elements using these liquid-crystal panels is formed by applying sheets to the opposite sides of the liquid-crystal display panel oriented with their polarization axes crossed at 90 degrees. In the case of the VA-type and IPS-type liquid-crystal panels, with respect to the viewing angle characteristics, maximum contrast can be obtained along the direction of the polarizing axis of the polarizing sheet, so that the sheets have polarizing axes oriented in parallel with the longitudinal or transverse direction of the liquid-crystal panel from the technical view point of symmetry of the viewing angle characteristics and visibility. Thus, these sheets to be applied to the liquid-crystal panel has a feature that the optical film including a polarizing composite film which has been subjected to a longitudinal or transverse stretching can be continuously fed out from a roll and cut along transverse lines with respect to the feed direction of the optical film to sequentially produce rectangular polarizing sheets including the polarizing sheets having the same width as the optical film width.
Because of the improved viewing angle characteristics, VA-type liquid-crystal or the IPS-type liquid-crystal are more widely adopted than the TN type. In view of such trend in environments of technical developments, proposals have been made such as the one described in Japanese Laid-Open Patent Publication No. 2004-361741A which is based on use of the VA-type or IPS-type liquid-crystal panels and comprises steps of continuously feeding an optical film laminate comprising a polarizing composite film, cutting the optical film laminate in conformity to the size of a liquid-crystal panel and sequentially laminating a plurality of sheets which have been produced by the cutting step to respective ones of a plurality of the liquid-crystal panels.
However, the mainstream of manufacture of liquid-crystal display elements is still based on the manufacturing technology utilizing individualized sheets, due to the following technical problems. In manufacturing liquid-crystal display elements, a critical technical challenge is to detect any defect which may otherwise be retained in the display elements to be formed, and to prevent any defective product from being produced. This makes it possible to significantly improve manufacturing yield. Most of the product defects primarily arise from defects in the polarizing composite film contained in the optical film. However, it is not practical to provide an optical film after completely removing all defects contained in individual films which are to be laminated together to form the optical film. The reason is that, observation for defects in the polarizing composite film on all of the polarizer and the protection film laminated on the polarizer to provide a polarizing composite film having no adhesive layer formed thereon, and an adhesive layer formed on the polarizing composite film indicates that there are various kinds of defects, including defects inherent in the PVA film of the polarizer itself, defects arose in connection with the lamination of the protection film to the polarizer and defects generated in the adhesive layer of the formed polarizing composite film, distributed in 20 to 200 positions over a unit length of the polarizing composite film of 1000 m. Thus, it is extremely difficult to produce a defect-free optical film under existing circumstances. To maintain quality of display elements, it is not permitted to use a polarizing composite film sheet having visible flaws or defects for a sheet for television display element even if such a flaw or defect is small. Therefore, if lengths of the polarizing composite film with defects are used to form a display and a display requires 1 m of film, 20 to 200 defective liquid-crystal display elements out of 1,000 products will be produced.
A proposed preliminary inspection apparatus for a polarizing composite film, is disclosed, for example, in Japanese Patent No. 3974400B and Japanese Laid-Open Patent Publications Nos. 2005-62165A and 2007-64989A.
Japanese Laid-Open Patent Publication 2007-140046A discloses a method that comprises peeling a carrier film from an optical film fed out continuously from a continuous roll to expose a polarizing composite film having an adhesive layer, detecting a defect or defects present in the polarizing composite film, punching or cutting only normal regions of a polarizing composite film into a rectangular shape, while leaving the defective region or regions of the polarizing composite film untouched. Japanese Patent Application No. 2007-266200 is a disclosure relating to a method and a system for laminating a polarizing sheet to a liquid-crystal panel. However, the method and system disclosed cause not only substantial complexity in the entire system for laminating but also an increase in the number of steps and difficulty in control for each step, and therefore, cause corresponding reduction in the manufacturing speed.
The present disclosure has been made based on the above related disclosures and through intensive researches and considerations for significantly enhancing product accuracy and manufacturing speed, and drastically improving manufacturing yield, in the manufacture of liquid-crystal display elements.